Search

This paper proposes a mathematics-informed neural network (MINN) approach for resolving the long-term challenge in wave scattering modeling. The central innovation lies in integrating Cauchy-Riemann equations into machine learning architectures. By incorporating Cauchy integrals and boundary conditions, the neural network successfully learns to numerically produce matrix kernel factorization for Wiener-Hopf analytical models. To validate and demonstrate the approach, a benchmark case of wave scattering from parallel hard-soft plates is studied by comparing the machine learning results with the available analytical solutions. The proposed MINN approach could provide a new route to extensively enhance the theoretical modeling capability for several wave scattering and fluid mechanics problems. The code can be found at https://github.com/lscapku/MINN. [ABSTRACT FROM AUTHOR]

A prediction method based on the information data of finite scattering acoustic pressure is proposed for the prediction of underwater spherical target multistatic acoustic scattering sound field, which represents the scattering sound field as the product of the acoustic scattering transfer function and the source density function, obtains the acoustic scattering transfer function by numerical integration with the model surface grid information as input, and acquires the source density function by combining acoustic reciprocity, least squares, and matrix transformation with small amount of known scattered sound pressure data as input. The multistatic scattering sound field prediction method is validated by using the Rayleigh normal mode solution and the finite element method (FEM) for double-layered spherical shells and spherical shell with the.pngfener. The target strength experiment is conducted for the spherical shell with the.pngfener, and the experiment results are compared with the finite element calculations. Several test data and FEM calculation results data are used as input to predict the multistatic scattering sound field of the spherical shell with.pngfener and compared with the FEM results, respectively. It is found that the simulation and test results are in good agreement, which verifies the feasibility of the prediction method.

This study focuses on investigating the propagation of acoustic waves in a cylindrical waveguide with higher‐order derivative‐based boundary conditions and a sound absorbent lining. The mode‐matching method is employed to solve the governing boundary value problem, utilizing continuity conditions for pressure and velocity at waveguide interfaces. The objective is to establish a theoretical framework for analyzing and optimizing the acoustic performance of the waveguide, considering the influence of crucial parameters. The study's findings hold significant practical implications in acoustic engineering and noise control, enabling accurate prediction of acoustic wave behavior and facilitating the design of effective noise reduction measures and optimized sound‐absorbing materials in various applications. Additionally, the numerical techniques developed here can be extended to address more complex waveguide geometries and boundary conditions. [ABSTRACT FROM AUTHOR]

In this study, the elastic properties of porous cylinders are evaluated using a nondestructive method based on ultrasonic wave propagation. In the scientific community, the elastic properties of materials play a crucial role. Measuring these properties can reveal valuable details about the mechanical properties of these materials. The scattering of a plane acoustic wave has received a great deal of attention from researchers due to its growing interest in many disciplines. Many authors have studied the acoustic scattering by means of cylindrical components (e.g. tubes, cylinders, etc.). This study shows how to analyze the distribution of circumferential waves with increasing porosity using the modal view of the waves propagating around the circumference of a cortical bone. This technique is applied on a signal backscattered by an porous tube of radius ratio b/a = 0.7 (a is the external radius and b is the internal radius). Our investigation revealed a correlation between bone porosity, fluid saturation, and pore radius, and changes in elastic properties. The Schoch model was developed to investigate the propagation of ultrasonic waves in porous bone. The results demonstrate the effectiveness of this approach for acoustic characterization and describing the impact of osteoporosis on bone elasticity. [ABSTRACT FROM AUTHOR]

This article provides a rigorous mathematical analysis of acoustic wave scattering induced by a high-contrast subwavelength resonator whose material density is periodically modulated in time, and with a modulation frequency that is much larger than the one of the incident wave. We find that in general, the effect of the fast modulation is averaged over time and that the system behaves as an unmodulated resonator with an apparent effective density. However, under a suitable tuning of the modulation, which achieves a matching between temporal Sturm-Liouville and spatial Neumann eigenvalues, the low frequency incident wave becomes suddenly able to excite high frequency modes in the resonator. This phenomenon leads to the generation of scattered waves carrying high frequency components in the far field, and to the existence of exponentially growing outgoing modes. From these findings, it is expected that such time-modulated system could serve as a spontaneously radiating device, or as a high harmonic generator. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Shanghai Jiao Tong University (1006-2467) is the property of Journal of Shanghai Jiao Tong University Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The scattering of fluid-structure-coupled waves is investigated in a cylindrical waveguide consisting of flexible shells and an expansion chamber connected via annular rigid discs. The scattering performance of the waveguide is studied in the presence of acoustic liners along the walls of the expansion chamber and the geometric variations along interfaces. A mode-matching method is applied to solve the corresponding mathematical problem. The study focuses on fluid- and structure-borne modes propagating in the flexible shells. The eigenfunctions in this case are non-orthogonal and obey generalized orthogonal properties, involving auxiliary parameters related to the ring conditions at the edges of the flexible shells. Although we considered clamped connections, the analysis can be extended to other ring conditions with appropriate changes. Through continuity conditions at the interfaces, orthogonal modes are used to project solutions in liner regions, resulting in linear algebraic systems. The performance of the waveguide is examined in terms of scattering powers and transmission loss as functions of frequency, chamber length, liner length, and porous screen impedance. It is observed that the variations in these parameters have a major influence on acoustic attenuation in the waveguide. These results are relevant for designing acoustic enclosures for different frequency bands.

In this paper, we establish new results for the uniform far-field asymptotics of the two-layered Green function (together with its derivatives) in two dimensions in the frequency domain. To the best of our knowledge, our results are the sharpest yet obtained. The steepest descent method plays an important role in the proofs of our results. Further, as an application of our new results, we derive the uniform far-field asymptotics of the scattered field to the acoustic scattering problem by buried obstacles in a two-layered medium with a locally rough interface. The results obtained in this paper provide a theoretical foundation for our recent work, where direct imaging methods have been developed to image the locally rough interface from phaseless total-field data or phased far-field data at a fixed frequency. It is believed that the results obtained in this paper will also be useful on its own right. [ABSTRACT FROM AUTHOR]

Purpose: Acoustic multi-connected domains are common in floating platforms and underwater vehicles. In order to improve the calculation efficiency of the traditional boundary element method and avoid dealing with complex connected domain boundary conditions, a method combining finite element method with artificial boundary element method is proposed to calculate the acoustic scattering of multi-domain structures. Methods: In this work, based on near-field artificial boundary, the multi-domain fluid is divided into simple exterior domain and complex multi-connected interior domain. The FEM of Software Nastran is easy to employ for structures and complex multi-domain fluids. The artificial boundary element method is used to generate the exterior added fluid mass, added fluid damping and pressure load associated with incident wave. The codes developed by the author and DMAP language are used to generate the matrixes and to couple the FEM with artificial BEM. Results: The correctness and convergence of the proposed numerical method are further investigated. The simulation results are in good agreement with theoretical data and literature results. When the scale of the artificial boundary element is less than one tenth of the wavelength of sound wave, the calculated solutions tend to be stable. In the case of cavity scattering, compared with the traditional boundary element method, the calculation time of proposed method is reduced by nearly half with the same calculation accuracy. Conclusion: Furthermore, the numerical method presented in this paper can solve scattering problems involving fluid spheres, bubbles, and multi-body structures, as well as the reflection of free water surfaces. [ABSTRACT FROM AUTHOR]

The scattering of fluid-structure-coupled waves is investigated in a cylindrical waveguide consisting of flexible shells and an expansion chamber connected via annular rigid discs. The scattering performance of the waveguide is studied in the presence of acoustic liners along the walls of the expansion chamber and the geometric variations along interfaces. A mode-matching method is applied to solve the corresponding mathematical problem. The study focuses on fluid- and structure-borne modes propagating in the flexible shells. The eigenfunctions in this case are non-orthogonal and obey generalized orthogonal properties, involving auxiliary parameters related to the ring conditions at the edges of the flexible shells. Although we considered clamped connections, the analysis can be extended to other ring conditions with appropriate changes. Through continuity conditions at the interfaces, orthogonal modes are used to project solutions in liner regions, resulting in linear algebraic systems. The performance of the waveguide is examined in terms of scattering powers and transmission loss as functions of frequency, chamber length, liner length, and porous screen impedance. It is observed that the variations in these parameters have a major influence on acoustic attenuation in the waveguide. These results are relevant for designing acoustic enclosures for different frequency bands. [ABSTRACT FROM AUTHOR]

A model of the acoustic scattering of a plane wave incident obliquely upon an infinite elastic bi-layered cylindrical shell is developed based on normal mode expansion technique. The far field backscattering form function for the composite copper/stainless steel cylindrical shell is calculated for reduced frequency range of k ⊥ a 1 = 0 – 70 and for different values of incident angles α (k ⊥ = k 1 cos ⁡ α , k 1 is the wave number in the fluid surrounding the shell and a 1 is the outer radius of the shell). Numerical calculations show the behaviour of the resonances of circumferential and helical guided waves as the angle of incidence increases. The obtained results are compared with those of copper and stainless steel one-layered cylindrical shells. The use of frequency-angle representation depicts the helical propagation of surface waves along the axis of the shell. This representation, along with the identification plan, enables to describe the evolution of acoustic scattering as a function of incidence angle and vibration mode n. Furthermore, a comparative analysis of the resonance trajectory evolution between mono-layered and composite cylindrical shells is performed. The evolution of the helical guided waves, the bifurcation of the A 0 wave into the A 0 − and A 0 + waves, and a region with high backscattering are investigated. The presented results show that the extension of the region with high backscattering depends on the thickness and physical properties of the bi-layered cylindrical shell. • Model of oblique acoustic scattering in bi-layered cylindrical shell (normal mode expansion). • Analysis of acoustic scattering in composite bi-layered shell. • Comparative study: mono-layered vs. bi-layered cylindrical shell. • Study resonance trajectory via frequency-angle & identification plan representations. • Estimation of cut-off angles, coupling angles & coincidence frequencies of guided waves. [ABSTRACT FROM AUTHOR]

The construction of wind farm pilings, submarine pipelines, and underwater submarines involves multiple cylinders. However, there is currently a lack of economic research on predicting the mechanism and characteristics of mutual coupling of acoustic scattering from multiple cylindrical targets. This study investigates the mechanism and prediction method of acoustic scattering for the structural distribution characteristics of underwater multi-cylindrical targets. A model of a multi-cylindrical target's two-dimensional acoustic field was established using the finite element method. Numerical calculations were then carried out to elucidate the scattering characteristics of the frequency–angle spectrum in far-field omnidirectional scattering. The simulation of echoes in the time domain explains how echoes propagate and interact with each other, and provides formulas for calculating interference and resonance frequencies. The frequency calculation formula extracts key features from the spectrum, providing a basis for predicting the characteristics of multi-cylindrical targets in terms of scale and spatial position. Measurement experiments were conducted on a double-cylindrical target in a water tank, and the theoretical calculations and experimental data were used to estimate the target's radius and distance. The actual layout confirms the accuracy of the interference and resonance frequency prediction formulas. This study offers a valuable solution for refined feature extraction and spatial estimation of underwater targets. [ABSTRACT FROM AUTHOR]

We study frequency domain acoustic scattering at a bounded, penetrable, and inhomogeneous obstacle Ω − ⊂ R d , d = 2 , 3 . By defining constant reference coefficients, a representation formula for the pressure field is derived. It contains a volume integral operator, related to the one in the Lippmann–Schwinger equation. Besides, it features integral operators defined on ∂ Ω − and closely related to boundary integral equations of single-trace formulations (STF) for transmission problems with piecewise constant coefficients. We show well-posedness of the continuous variational formulation and asymptotic convergence of Galerkin discretizations. Numerical experiments in 2D validate our expected convergence rates. [ABSTRACT FROM AUTHOR]

In this work, a two-dimensional domain decomposition technique (DDT) is developed based on an overset method that couples a nonlinear Euler solver with a linearized Euler solver. The nonlinear domain accounts for complex physics in the near field, such as acoustic wave interaction with shock waves and acoustic wave scattering from solid bodies. The linear domain allows the small perturbation waves induced by the nonlinear effects to propagate with minimal numerical dissipation and dispersion while also significantly reducing computational resources. To this end, two scenarios have been considered: (i) wave amplification and modulation in a converging-diverging duct with a standing shock wave and (ii) wave scattering from an incident acoustic wave over a supercritical airfoil. With DDT, these cases show a reduction in computational time by more than 70% compared to their respective fully-nonlinear simulations. The accuracy of the simulations with DDT is found to depend on the extent of overlap between the linear and nonlinear zones. In general, an overlap ratio of at least two is necessary to maintain high order of convergence rate, though still limited by the nonlinear scheme. Furthermore, the implementation of DDT in the external flow case can eliminate numerical noise observed at the far field of the fully-nonlinear counterpart, which do not have any effective non-reflecting boundary treatment. [ABSTRACT FROM AUTHOR]

In this study, we focus on the prediction of the pressure field scattered from an immersed cylindrical shell partially coated by a soft rubber, impacted by an acoustic plane wave. As the coating covers only a partial portion along the circumference of the shell, the considered system is not axisymmetric. As a result, a spectral (Fourier) resolution of the mathematical problem would induce the coupling of the different circumferential orders, which can lead to prohibitive computing times. To circumvent this issue, the reverse Condensed Transfer Function (rCTF) method has recently been developed to decouple vibroacoustic subsystems initially coupled along lines or surfaces. From an analytical model of the fully coated shell impacted by the acoustic plane wave and a finite element (FE) model of the missing coating material, the rCTF approach predicts the vibroacoustic behavior of the coated shell with a voided section instead of the removed part. This voided section can then be filled by a FE model of the water domain replacing the removed coating material, using the direct CTF approach. The principle of the rCTF approach, some numerical validations, and results for the scattering from the partially coated shell are presented in this paper. [ABSTRACT FROM AUTHOR]

It is vital to consider acoustic scattering when using geometrical acoustic simulation techniques, such as ray tracing. However, there are few methods for modelling scattering, and most rely on strong assumptions of uniformity on the distribution of scattered energy. In this paper, a model for directional scattering in ray tracers is presented. The model is based on an idealized model of a 1D scatterer, which is then used to extend the most commonly used scattering algorithm in ray tracers today. The developed algorithm is implemented in a ray tracer and tested to evaluate its performance compared to existing methods. It is found that the directional scattering algorithm can be used to replicate measured effects on room acoustic parameters caused by changes in the orientation of 1D scatterers. [ABSTRACT FROM AUTHOR]

It is vital to consider acoustic scattering when using geometrical acoustic simulation techniques, such as ray tracing. However, there are few methods for modelling scattering, and most rely on strong assumptions of uniformity on the distribution of scattered energy. In this paper, a model for directional scattering in ray tracers is presented. The model is based on an idealized model of a 1D scatterer, which is then used to extend the most commonly used scattering algorithm in ray tracers today. The developed algorithm is implemented in a ray tracer and tested to evaluate its performance compared to existing methods. It is found that the directional scattering algorithm can be used to replicate measured effects on room acoustic parameters caused by changes in the orientation of 1D scatterers.

The present study examines the fluid-coupled wave response of a stepped elastic membrane discontinuity connecting a flexible shell to the annular rigid shell. The acoustic radiation from the internal and annular regions propagates along the flexible boundaries as well as through the compressible fluid that fills the interior of the shell and scatters at geometric discontinuity and edges. In contrast to previous studies, an elastic membrane disc with different boundary conditions is considered as the geometric discontinuity rather than an acoustically rigid step-discontinuity in a cylindrical waveguide. The problem governed by the rigid step-discontinuity is solved using the mode matching technique, while the Galerkin procedure is used for the elastic membrane discontinuity. The truncated form of both solutions reconstructs the matching conditions and satisfies the conservation laws, which further authenticate the correctness of the solutions and the validity of the obtained numerical results based on these solutions. It is worth noting that the parametric configuration of the elastic membranes, the edge conditions and the radii of the shells have a significant impact on the attenuation of the noise control device. [ABSTRACT FROM AUTHOR]

Using the tunderwater corner reflector (CR) to simulate the acoustic scattering characteristics of the military target is a new technology to counter active sonar detection. Existing underwater CRs only have the ability to interfere with the acoustic field, but have limitations in acoustic wave modulation. Therefore, acoustic metasurfaces applied on CRs to enhance the ability of acoustic wave modulation has a great application prospect. A fast prediction method based on the Kirchhoff approximation (KA) and the ray tracing theory is proposed to calculate the acoustic scattering characteristics of CR with acoustic metasurfaces in grooves array type. The accuracy of the method is verified by the finite element method (FEM) simulation. The modulation effect of CR with grooves array in different gradient combinations on the structural scattering acoustic field is analyzed. The research shows that the CR with different combinations of the acoustic metasurface has an obvious modulation effect on the amplitude of the acoustic waves and the deflection of acoustic field. In particular, the grooves array in combination with positive and negative gradients has an obvious deflection impact on the scattering acoustic field. [ABSTRACT FROM AUTHOR]

We propose and study a method for finding quasi-resonances for a linear acoustic transmission problem in frequency domain. Starting from an equivalent boundary-integral equation we perform Galerkin boundary element discretization and look for the minima of the smallest singular value of the resulting matrix as a function of the wave number k. We develop error estimates for the impact of Galerkin discretization on singular values and devise a heuristic adaptive algorithm for finding the minima in prescribed k-intervals. Our method exclusively relies on the solution of eigenvalue problems for real k, in contrast to alternative approaches that rely on extension to the complex plane. [ABSTRACT FROM AUTHOR]

Bubbles are widely used in the medical field due to their strong acoustic scattering properties, and the interaction between bubbles affects the scattering acoustic field caused by the bubble cluster. In this study, the dynamic equations of bubbles oscillating in viscoelastic tissues are solved numerically. The effect of bubble interaction on the scattered acoustic pressure under dual-frequency ultrasound is analyzed. In addition, the frequency spectra of the scattered acoustic waves due to the bubbles with and without considering the interaction are compared. The results show that the suppression or enlargement of the scattered sound pressure caused by the interaction between bubbles is related to the bubble radius and the incident frequency. Moreover, when the incident frequency is equal to the resonant frequency of the bubble with equilibrium radius R0, the effect of resonant bubbles is stronger than that of non-resonant bubbles. Meanwhile, for the multi-bubble system with a small bubble number density, the total response of the bubble cluster can be approximated as an algebraic sum of the dynamical behavior of individual bubbles.

Based on the perfectly matched layer (PML) technique, this paper develops a highly accurate boundary integral equation (BIE) solver for acoustic scattering problems in locally defected layered media in both two and three dimensions. The original scattering problem is truncated onto a bounded domain by the PML. Assuming the vanishing of the scattered field on the PML boundary, we derive BIEs on local defects only in terms of using PML-transformed free-space Green's function, and the four standard integral operators: single-layer, double-layer, transpose of double-layer, and hyper-singular boundary integral operators. The hyper-singular integral operator is transformed into a combination of weakly-singular integral operators and tangential derivatives. We develop a highorder Chebyshev-based rectangular-polar singular-integration solver to discretize all weakly singular integrals. Numerical experiments for both two- and three-dimensional problems are carried out to demonstrate the accuracy and efficiency of the proposed solver. [ABSTRACT FROM AUTHOR]

A surface homogenization method for acoustic waves over thin microstructured surfaces in the presence of a fluid in a potential flow is presented. Sound hard surfaces are considered, the flow is considered two-dimensional and slow and a low Mach approximation is introduced. We consider acoustic waves with a typical wavelength 1/k much larger than the array spacing h and thickness e. Owing to the small parameter ε=kh , with e/h=O(1) , a matched asymptotic expansion technique is applied to the low Mach potential wave equation in the frequency domain. A boundary condition is obtained on an equivalent flat wall, which links the acoustic velocity to its normal and tangential derivatives (of the Myers type). The accuracy of the effective model is tested numerically for various periodic shapes and the accuracy of the model in O(ε2) is validated. [ABSTRACT FROM AUTHOR]

A generative deep learning approach for shape recognition of an arbitrary object from its acoustic scattering properties is proposed and demonstrated. The strategy exploits deep neural networks to learn the mapping between the latent space of a 2D acoustic object and the far‐field scattering amplitudes. A neural network is designed as an adversarial autoencoder and trained via unsupervised learning to determine the latent space of the acoustic object. Important structural features of the object are embedded in lower‐dimensional latent space which supports the modeling of a shape generator and accelerates the learning in the inverse design process. The proposed inverse design uses the variational inference approach with encoder‐ and decoder‐like architecture where the decoder is composed of two pretrained neural networks: the generator and the forward model. The data‐driven framework finds an accurate solution to the ill‐posed inverse scattering problem, where nonunique solution space is overcome by the multifrequency phaseless far‐field patterns. This inverse method is a powerful design tool that doesn't require complex analytical calculation and opens up new avenues for practical realization, automatic recognition of arbitrary‐shaped submarines or large fish, and other underwater applications. [ABSTRACT FROM AUTHOR]

The current study models and explains the fluid–structure coupled response of a flexible shell connected with two rigid coaxial shells via circular step discontinuities. Radiations from annular and inner regions of inlet rigid shells scatter on interfaces after interaction with the geometric variations and change of material properties. The flexible shell that supports vibrations along the shell is expressed though the Donnell–Mushtari formulation in the differential system. The mode‐matching technique is applied to solve the governing boundary value problem. The solution is projected on the orthogonal characteristics which in flexible casing is generalized as compared with the properties of acoustically rigid setting. The implication of generalized orthogonality relations governs additional constants that are found by defining the physical connection of shell with rigid disc. The clamped and pin‐jointed conditions are considered in this article. The scattering powers and transmission loss are analyzed against frequency, chamber length, and radii of shells for clamped and pin‐jointed edge conditions. It is found that the power propagation in annular shell and inner shell behave conversely. Moreover, configuration can be attenuated through the variation of dimensions of chamber and edge conditions. [ABSTRACT FROM AUTHOR]

A half-space x2>0 is occupied by a two-component statistically uniform random composite with specified volume fractions and two-point correlation. It is bonded to a uniform half-space x2<0 from which a plane wave is incident. The problem requires the specification of the properties of three media: those of the two constituents of the composite and those of the homogeneous half-space. The complexity of the problem is minimized by considering a model acoustic-wave problem in which the two media comprising the composite have the same modulus but different densities. The homogeneous half-space can have any chosen modulus and density. An approximate formulation based on a stochastic variational principle is formulated and solved explicitly in the particular case of an exponentially decaying correlation function, generalizing a previous solution in which the homogeneous medium had the same modulus as the composite. The main novelty of the present work is that the mean energy fluxes in the composite and reflected from the boundary are calculated, demonstrating explicitly the large backscatter associated with the mean-zero component of the reflected signal and the systematic transfer of energy from the decaying mean transmitted waves into the mean-zero disturbance as distance from the boundary increases. [ABSTRACT FROM AUTHOR]

In this study, the T-matrix method combined with the addition theorems of spherical basis functions is applied to semi-analytically compute the underwater far-field acoustic scattering of a pair of rigid spheroids with arbitrary incident angles. The involvement of the addition theorems renders the multiple scattering fields of each spheroid to be translated into an identical origin. The accuracy and convergence property of the proposed method are verified and validated. The interference of specular reflection wave and Franz wave can be spotted from the oscillations of the form function. Furthermore, the propagation paths of specular reflection and Franz waves are quantitatively analyzed in the time domain with conclusions that the Franz waves reach the observation point subsequent to specular reflection waves and the time interval between these two wave series is equal to the time cost of the Franz waves traveling along the sphere surfaces. Finally, the effects of separation distances, aspect ratios (the ratio of the polar radius to equatorial radius), non-dimensional frequencies, and incidence angles of the plane wave on the far-field acoustic scattering of a pair of rigid spheroids are studied by the T-matrix method.

The construction of wind farm pilings, submarine pipelines, and underwater submarines involves multiple cylinders. However, there is currently a lack of economic research on predicting the mechanism and characteristics of mutual coupling of acoustic scattering from multiple cylindrical targets. This study investigates the mechanism and prediction method of acoustic scattering for the structural distribution characteristics of underwater multi-cylindrical targets. A model of a multi-cylindrical target’s two-dimensional acoustic field was established using the finite element method. Numerical calculations were then carried out to elucidate the scattering characteristics of the frequency–angle spectrum in far-field omnidirectional scattering. The simulation of echoes in the time domain explains how echoes propagate and interact with each other, and provides formulas for calculating interference and resonance frequencies. The frequency calculation formula extracts key features from the spectrum, providing a basis for predicting the characteristics of multi-cylindrical targets in terms of scale and spatial position. Measurement experiments were conducted on a double-cylindrical target in a water tank, and the theoretical calculations and experimental data were used to estimate the target’s radius and distance. The actual layout confirms the accuracy of the interference and resonance frequency prediction formulas. This study offers a valuable solution for refined feature extraction and spatial estimation of underwater targets.

The description of acoustical waves can be achieved using an expansion over basic functions with weighting coefficients which may be called beam shape coefficients (BSCs). There is a strong analogy between the scalar formalism of acoustical waves and the vectorial electromagnetic formalism, known as generalized Lorenz–Mie theory (GLMT), describing the interaction between a homogeneous sphere and an arbitrary illuminating beam. In particular, BSCs have been introduced as well in GLMT and the mathematical arsenal to evaluate them, developed since several decades, can be used mutatis mutandis to evaluate BSCs in acoustics. In particular, the present paper introduces a method named localized approximation to the evaluation of the acoustical BSCs, similar to the localized approximation used to evaluate electromagnetic BSCs, in the case of Bessel beams. Such a formalism akin to the electromagnetic GLMT may be viewed as an acoustical GLMT and should allow a renewal of the calculation of various properties of acoustical wave scattering, in particular to the design of acoustical tweezers similar to optical tweezers.

The acoustic radiation and scattering of underwater bubbles play an important role in ocean exploration, target localization, acoustic measurements, etc. The two-phase fluid flow and moving boundary result in the lack of means to predict the sound field, which limits the exploration of relevant characteristics and mechanisms. The present work is intended to introduce a numerical approach for acoustic radiation and scattering of moving bubbles at low frequencies with CFD-BEM coupling method. The two-phase interface is captured with the volume of fluid scheme and the sound field is solved with the boundary element method. After that, some benchmark problems are solved and the results are compared with data from literatures. Finally, the radiation and scattering of moving bubbles at low frequencies are predicted with our approach. The acoustic radiation pressure of bubbles shows a slight increase trend during deformation. As the ka value increases, both the acoustic directionality of radiation and scattering exhibit main and side lobes, and the scattering energy gradually concentrates in the positive direction of the incident wave. For a moving bubble, its displacement and velocity of moving have a significant impact on the directionality of the scattered sound field. Therefore, the problem of bubble localization can be studied based on directionality shift.

A generative deep learning approach for shape recognition of an arbitrary object from its acoustic scattering properties is proposed and demonstrated. The strategy exploits deep neural networks to learn the mapping between the latent space of a 2D acoustic object and the far‐field scattering amplitudes. A neural network is designed as an adversarial autoencoder and trained via unsupervised learning to determine the latent space of the acoustic object. Important structural features of the object are embedded in lower‐dimensional latent space which supports the modeling of a shape generator and accelerates the learning in the inverse design process. The proposed inverse design uses the variational inference approach with encoder‐ and decoder‐like architecture where the decoder is composed of two pretrained neural networks: the generator and the forward model. The data‐driven framework finds an accurate solution to the ill‐posed inverse scattering problem, where nonunique solution space is overcome by the multifrequency phaseless far‐field patterns. This inverse method is a powerful design tool that doesn't require complex analytical calculation and opens up new avenues for practical realization, automatic recognition of arbitrary‐shaped submarines or large fish, and other underwater applications.

The characterization of targets by electromagnetic (EM) scattering and underwater acoustic scattering is an important object of research in these two related fields. However, there are some difficulties in the simulation and measurement of the scattering by large targets. Consequently, a similarity study between acoustic and EM scattering may help to share results between one domain and the other and even provide a general reference method for the simulation of scattering characteristics in both fields. Based on the method of physical optics, the similarity between the EM scattering of conductors and the acoustic scattering of soft/hard targets and the similarity between the EM scattering of dielectrics and the acoustic scattering of elastics are studied. In particular, we derive how to transfer quantities from one domain into another so that similar scattering patterns arise. Then, according to these transfer rules, the EM scattering and acoustic scattering of three typical targets with different types of boundaries were simulated and measured, and the simulated EM scattering and acoustic scattering curves were found to be in perfect agreement, with correlation coefficients above 0.93. The correlation coefficients between the electromagnetic and acoustic scattering patterns were above 0.98, 0.91, and 0.65 for three typical targets. The simulated and measured scattering results verify the proposed similarity theory of EM and acoustic scattering, including the transfer from one domain into the other and the conditions of EM and acoustic scattering, and illustrate that the acoustic scattering characteristic of the target can be simulated using the EM scattering based on the derived conditions and vice versa. [ABSTRACT FROM AUTHOR]

We extend the fictitious domain spectral method presented in Gu and Shen (SIAM J Sci Comput 43:A309–A329, 2021) for elliptic PDEs in bounded domains to the Helmhotlz equation in exterior domains. We first reduce the problem in an exterior domain to a bounded domain using the exact Dirichlet-to-Neumann operator. Next, we formulate the reduced problem into an equivalent problem in an annulus by using a fictitious domain approach. Then, we apply the Fourier-spectral method in the radial direction to reduce the problem in an annulus to a sequence of 1-D Bessel-type equations, each with a one-sided open boundary condition that are to be determined by the boundary condition of the original Helmholtz equation. We solve these 1-D Bessel-type equations by the Legendre-spectral method, and determine the open boundary conditions with a least square approach. We derive a wave number explicit error estimate for the special case of a circular obstacle, and provide ample numerical results to show the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Rapid calculation of large-scale acoustic scattering from complex targets is the basis of many acoustic technologies. The key issue is to efficiently solve the dense and ill-conditioned linear equations caused by semi-analytical boundary collocation method. This article aims to establish a dual-level fast direct solver to replace the GMRES for solving such a large-scale dense linear system. The core idea of the dual-level fast direct solver is to construct a sparse approximate inverse matrix to indirectly approximate the solution of linear equations. The solution process consists of the smoothing process and the correction process. Therefore, the dual-level fast direct solver only needs to solve a small-scale coarse-mesh matrix and a series of small-scale least squares problems defined on fine mesh. The drawback that the GMRES cannot efficiently solve large-scale dense linear system is overcome. Several practical engineering problems are analyzed by the dual-level fast direct solver, such as the acoustic scattering from an A320 aircraft and acoustic scattering from a human head. [ABSTRACT FROM AUTHOR]

It may happen that under a certain wave interrogation, a medium scatterer produces no scattering. In such a case, the scattering field is trapped inside the scatterer and forms a certain interior resonant mode. We are concerned with the behavior of the wave propagation inside a transparent scatterer. It turns out that the study can be boiled down to analyzing the interior transmission eigenvalue problem. For isotropic mediums, it is shown in a series of recent works that the transmission eigenfunctions possess rich patterns. In this paper, we show that those spectral patterns also hold for anisotropic mediums. [ABSTRACT FROM AUTHOR]

• Six DOFs of warship under different sea states can be efficiently calculated by strip theory. • Random reflection coefficients close to real environment can be obtained by empirical formula and Monte Carlo method. • Accurately predicting echo strength of warships coupled with undulating sea surfaces. • Echo strength of warship with wave dynamic coupling conforms to the statistical regularity of Chi-Square distribution. The echo signatures of a warship can be tracked by an underwater weapon equipped with an active sonar system. However, the dynamics of undulating sea surfaces and the influence of sound reflection result in a lack of sufficiently detailed feature studies. A hybrid method is proposed to predict the acoustic scattering of warships under time-varying undulating sea surfaces, considering the multiple reflection effects of the sea surface. Specifically, the echo strength (ES) of warships is predicted under various wave spectra by coherently summing the echoes from different paths using the Four-Path method. This predictive process involves constructing a 3D warship model, acquiring the six degrees of freedom (DOFs) motion attitude, and determining the sea surface reflection coefficient under different sea state parameters. Furthermore, a statistical model is proposed for the ES of undulating interface targets. To validate the effectiveness of this hybrid method, an acoustic scattering echo experiment was conducted in a towing tank under wave-making conditions. The results demonstrate that the average error of ES between the prediction and experimentation is less than 2 dB within the detection frequency range of 120 kHz to 180 kHz. With the statistical model, the results also indicate that the ES over time conforms to the statistical regularity of the chi-square distribution. Additionally, the mean value of ES and the value of double degree of freedom k align well. In summary, this work represents a systematic effort to predict the ES of warships coupled with undulating sea surfaces using a new method. [ABSTRACT FROM AUTHOR]

Partial wave expansions with spherical wave functions are usually employed to describe complex velocity potentials or pressure fields in ultrasonic and acoustic scattering by spherical particles. The expansion coefficients – known as the beam shape coefficients (BSCs) – can be calculated using different approaches which have been largely unexplored in the acoustical literature. Here, we formally present the Finite Series method for the evaluation of BSCs of ultrasonic and acoustic arbitrary-shaped beams. Such a series, which relies on Neumann expansion theorem, is an alternative to quadratures whenever analytical solutions cannot be obtained from direct integration over polar and azimuthal spherical angles. As examples of application, we consider the calculation of BSCs of Bessel, Laguerre–Gauss and Gaussian beams under on-axis configuration, including a discussion on a modified version of the finite series method recently presented in the context of optical fields. • A systematic approach to the finite series is presented for acoustic scattering. • The approach is valid for lossless and inviscid fluids. • Exact beam shape coefficients (BSCs) for beams satisfying the Helmholtz equation. • BSCs of paraxial beams provide approximate remodeling of the velocity potential. • Examples of are given for Bessel, Gaussian and Laguerre–Gauss ultrasonic beams. [ABSTRACT FROM AUTHOR]

Under oblique incidence acoustic wave, we investigate theoretically and numerically the dispersion curves of bilaminated cylindrical shell which composed of copper and stainless steel, immersed in water and filled with air. The investigation delves into the influence of the incidence angle on the phase and group velocities of guided waves. As the incidence angle increases, circumferential and helical guided waves are initiated parallel to the axis of the cylindrical shell. This initiation transpires at an incident angle denoted as α l = sin − 1 (C 1 / C l) (where C 1 represents the wave velocity in water and C l signifies the phase velocity of the l th guided wave). Beyond this angle, the respective guided wave undergoes a transition to a "cut off" state, adopting an evanescent character. This investigation also unveils that at a particular angle of incidence, resonance trajectories of certain guided waves manifest at cut-off frequencies, and these frequencies escalate with the angle of incidence. Concurrently, we examine how varying thicknesses impact the dispersion curves of waves in both the outer layer (copper) and inner layer (stainless steel) of the bilaminated shell. Notably, we observe a sensitivity in the phase and group velocities concerning changes in these thicknesses. Furthermore, the dispersion curves of the bilaminated cylindrical shell presents a tendency to shift towards low frequencies as either the outer or inner layer progressively thickens. This trend brings the dispersion curves closer to those of a single-layered cylindrical shell composed of the material with the greater thickness in the bilaminated shell. The obtained results demonstrate a commendable agreement with the existing experimental findings on copper and stainless steel single-layered cylindrical shells. [Display omitted] • Investigation of acoustic scattering by a bilaminated cylindrical shell under oblique excitation. • Estimation of cut-off reduced frequencies of certain guided waves. • Determination and analysis of dispersion curves of guided waves. • Effects of incidence angle and thickness on the guided waves velocities. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive evaluation of the near-field acoustics of an aircraft auxiliary power unit (APU), based on experimental data acquired from an in-situ APU. The aim is to establish whether near-field acoustics can be implemented for online condition monitoring. The APU of Cranfield University’s demonstrator aircraft, a Boeing 737-400, has been instrumented to acquire acoustics (near-field and far-field) and vibration data in synchronization with aircraft state parameters under a wide range of operating conditions. The acquired data is first implemented to determine the efficacy of employing near-field / far-field microphones, and vibration sensors, to monitor the combustion noise and tonal frequency levels from the APU components. Subsequently, an evaluation of the broadband characteristics of the vibroacoustic data and its variations against APU states and performance parameters is conducted based on several categories of feature extraction techniques. The findings suggest that nearfield acoustics lacks the ability to capture the combustion noise process. In addition, the tonal frequencies are also lost due to the level of background noise, fluctuations in the APU speeds, and scattering effects. For the same reasons, the phase couplings occurring between the signals generated by the APU components cannot be detected using acoustic data. Nevertheless, the overall analysis substantiates that the near-field acoustic data can be used to predict the APU operating states and has the potential to be implemented for developing APU performance parameter estimation models to enable condition monitoring.

We develop a framework shedding light on common features of coupled variational formulations arising in electromagnetic scattering and acoustics. We show that spurious resonances haunting coupled domain-boundary formulations based on direct boundary integral equations of the first kind originate from the formal structure of their Calderón identities. Using this observation, the kernel of the coupled problem is characterized explicitly, and we show that it completely vanishes under the exterior representation formula. [ABSTRACT FROM AUTHOR]

Acoustic and elastic metamaterials have shown potential for many different applications in wave manipulation and structural isolation. Minimization of a scattered wave field has been identified as an important use for such materials, and a wide variety of designs have been proposed for this purpose. Several of these designs face practical challenges (e.g. pentamode designs, which are difficult to manufacture), and many others involve numerous tunable parameters and a large design parameter space (e.g. distributions of resonators), making global search-based optimization less tractable and raising the need for gradient-based methods. In this work, gradient-based optimization governed by partial differential equations (PDEs) is used to optimize acoustic channels in a coiled-space metastructure layer surrounding a rigid acoustic scatterer. The optimization technique is chosen to more feasibly search the design parameter space, and the acoustic channel design is chosen to exploit sound speed optimization as a proxy for channel length optimization to give practically achievable structures. Results are presented for both a single plane wave direction and multiple plane wave directions, as well as for both a single frequency and multiple frequencies. The equivalence of sound speed optimization and channel length optimization is demonstrated for a feasible design minimizing the scattered pressure field. Designs with various numbers of optimized channels are also considered. [ABSTRACT FROM AUTHOR]